1. Field of the Invention
The present invention relates generally to the technology of transmission on optical fibers and more particularly to a reconfigurable directional optical system.
2. Description of the Prior Art
Using optical fibers for new communication networks has become the norm. Large quantities of optical fibers have already been installed, often, as shown in FIG. 1A, in the form of cables 110 combining a plurality of fibers 120. The cables interconnect optical communication equipment units, for example an optical cross-connect (OXC) system 100 for distributing traffic between the fibers of three cables 110, 112, 114 in the particular example shown in FIG. 1A.
A communication network based on optical fibers can include other optical devices. FIG. 1B shows a device that is often needed, namely an optical add/drop multiplexer (OADM) 130 for optically adding and dropping local traffic. In this case, the OADM drops the portion of the traffic incoming on one or more fibers 132 and intended for local use and adds traffic 136 generated locally. Thus the traffic outgoing on one or more fibers 138 comprises all the incoming traffic 132 less the traffic 134 dropped locally plus the traffic 136 generated locally.
Another type of device used in optical networks, shown in FIG. 1C, is a light amplifier 140, often of the erbium-doped fiber amplifier (EDFA) type, which amplifies the level of the incoming light signals 142. If the latter must be propagated over great distances, the amplifier produces a level at the output 144 sufficient to process them locally or to relay them to a more distant destination.
The reader cannot have failed to notice, on examining FIGS. 1A, 1B and 1C, that the various optical devices shown always use the installed fibers in the same propagation direction. Thus, in FIG. 1A, the four fibers of the cable 114 on the left-hand side of the figure propagate light signals toward the right in the case of two of the fibers and toward the left in the case of the other two fibers. The situation is similar for the other two cables 110, 112. Thus the resources in terms of fibers are allocated in a fixed manner, which has the consequence that the resources in one direction must be allocated for the peak traffic in that direction. Persons skilled in the art of communication networks are well aware that the traffic at a node of a network can be highly asymmetric, one direction having to carry a much greater quantity of information than the other. This is the case with video distribution networks in particular, which are beginning to be installed and in which the quantity of information to be distributed, in the form of pictures, is always infinitely greater than the quantity of information contained in initial requests received from users. Also, the asymmetry of the traffic can change with time, for example over a period of 24 hours. This is the case with international data communications, for example between Europe and North America, because of time differences.
Thus the fixed allocation of resources in terms of optical fibers of a network presupposes that they be dimensioned for the peak traffic in both directions even though the traffic peaks may never occur simultaneously, with the result that some of the resources are always unused.
This is why the object of the invention is to provide a directional optical system that is reconfigurable so that network resources in terms of optical fibers can be allocated dynamically.